1. Field of the Invention
The present invention relates to methodology for controlling the residual stresses in thin film materials formed by deposition on a substrate, and also relates to thin film structures which are characterized by reduced stress therein relative to thin films of corresponding materials produced by the prior art.
2. Description of the Related Art
In the prior art fabrication of thin film materials such as metal-containing films formed by vapor phase deposition processes, including metal oxides having use in semiconductor device applications, residual stresses in the deposited thin film material can render the film deficient or even useless for its intended function. In other cases, certain amounts of a tensile or compressive stress are beneficial.
Such residual stresses may for example interfere with desired properties of the product film, such as piezoelectric, paraelectric, superconducting, ferroelectric, and memory (in the case of memory alloy thin films) properties, and the residual stresses may compromise the structural integrity of the material, particularly in applications where the thin film is subjected to differential thermal effects or mechanical impact, vibration, etc.
For these reasons, it is desirable to form thin film materials in such manner as to minimize and/or otherwise control the residual stress in the product material.
The prior art has attempted to minimize the residual stresses in thin film materials by focusing largely on the selection and manipulation of the process conditions, using film-forming methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), spin-on formation, etc.
In addition to such process conditions approach, there has been recognition that the mechanical fixturing and physical set-up of the substrate element on which the thin film is formed may have significant impact on the stress state of the final product film.
For example, in discussing the highly compliant nature of PLZT (Pbxe2x80x94Laxe2x80x94Zrxe2x80x94Tixe2x80x94O), Haertling (G.H. Haertling, Ferroelectrics, 1987, Vol. 75, pp. 25-55, III. Properties, 2. Mechanical Properties, 2nd paragraph) has mentioned a device reported in J. Maldonado and A. Meitzler, Proc. IEEE, 1971, Vol. 59, p. 368, in which xe2x80x9cdomain orientation was accomplished by mechanically flexing a PLZT plate bonded to a plexiglas substratexe2x80x9d. This work evidences the fact that stress can be used to control domain orientation.
The present invention is therefore directed to an improved methodology to control the stress in thin film materials formed on a substrate, and to correspondingly improved thin film materials having enhanced stress characteristics relative to the films produced by the prior art.
In accordance with the present invention, there is provided an improved methodology for forming a thin film material on a substrate by deposition on a substrate of film-forming material therefor, in which the substrate is subjected to deformation or force to produce films of significantly improved character, relative to corresponding films produced by deposition on a substrate not subjected to such deformation or force.
In one aspect of the invention, the polarization direction of a ferroelectric film grown above the Curie temperature (in the paraelectric state), which can be significantly influenced by the state and magnitude of stress as the film becomes ferroelectric upon cooling below the Curie temperature, is formed on a substrate while the substrate is subjected to the application of force in a manner which opposes the stress evolution otherwise experienced in the cool-down from the film-forming temperature.
The method of the invention is advantageous in application to formation of films that undergo diffusionless phase transformations typically involving the displacement of atoms and a change in crystal symmetry. Examples of technologically important materials of such type include steel, memory alloys (NiTi and Cu-based alloys), dielectric materials such as BaSrTiO3, polymeric materials, and piezoelectric materials. For example, the method of the invention may be employed for oriented polymeric materials, e.g., piezoelectrics such as polyvinylidene difluoride (PVDF), useful as a piezoelectric and pyroelectric material, and potentially for other dielectric applications, such as low k value dielectrics.
The method of the invention is also applicable to the formation of films that undergo phase changes during processing where stress is usefully employed to control the crystallographic texture of the material. Nuclei will form in the parent phase in an orientation that minimizes the free energy of the system, and if the parent phase film is stressed, then the nuclei will tend to form with a soft direction in the plane of the film to minimize its strain energy. The soft direction of a crystalline film material is the crystallographic direction that is elastically more compliant than the other crystallographic directions of such material. For example, in a copper crystal, the  less than 100 greater than  family of directions is typically more compliant than are the  less than 111 greater than  directions.
The method of stress engineering in accordance with the invention is also usefully employed in a wide variety of materials fabrication applications, such as for example the formation on a silicon substrate of a sputtered metal film whose growth stress is large and compressive. Since the coefficient of thermal expansion (CTE) of the metal film is greater than the CTE of the Si substrate material, the stress in the film at room temperature can be reduced by sputtering at an elevated temperature. At the elevated deposition temperature, the film is still in compression, but as it cools on the substrate, it approaches a stress-free state. However, such elevated temperature film-formation conditions may be detrimental to other layers of an integrated circuit (IC) device. The same near-stress-free state can be obtained in accordance with the present invention by constraining the substrate during the sputter deposition and then releasing the constraint after deposition, so that the top surface of the substrate is given the amount of compressive strain as is needed to be released from the sputtered metal layer.
The methodology of the invention is also applicable in the converse to films that have little growth stress, but must be deposited at a high temperature because of the constraints of a CVD or other elevated temperature process. In such case, the thermal expansion mismatch strain can be compensated in the practice of the invention by constraining the substrate at the deposition temperature. In this way, there is little or no stress during deposition, and a stress is created during cooling, but the stress is then relieved by removing the wafer constraint.
The stress of a film at or away from the processing temperature often controls the desired properties of the film, which may include tribological, optical, thermal, magnetic, electrical, etc., properties. An important application of this problem is control of the crystalline orientation of perovskite thin films such as Pb(Zr,Ti)O3 (PZT), which are currently being developed for non-volatile memories, pyroelectric IR imaging arrays, high permittivity capacitor dielectrics and electro-optic spatial light modulators. For ferroelectric PZT, the important electrical and electro-mechanical properties are anisotropic, and for most applications there is an optimum crystalline orientation. The role of stress in PZT thin films, particularly as in determining the crystalline orientation of the perovskite film during cooling of a film through the Curie transition, is described in xe2x80x9cRelationships Between Ferroelectric 90xc2x0 Domain Formation and Electrical Properties of Chemically Prepared Pb(Zr,Ti)O3 (PZT) Thin Filmsxe2x80x9d, by Tuttle et al., Science and Technology of Electronic Materialsxe2x80x94NATO ASI Seriesxe2x80x94Series E: Applied Sciences, O. Auciello and R. Waser, Eds. vol. 284, p. 117, (1995).
The methodology of the invention is useful in a broad class of situations where there is some reason to control the stress in the film independent of other process conditions.
In many instances, it may be preferred to deposit the film without constraining the substrate, and then in accordance with the methodology of the invention, to constrain the substrate for some post-processing period in order to alter the properties of the film or to alter the way that the film develops under post-processing conditions.
In a specific aspect, the invention may be carried out with a wafer to the backside of which (opposite the side of the wafer on which the product film is being formed) is secured a film, plate, or other material element formed of a material of construction having a different CTE in relation to the wafer, so that the wafer is bent or deformed in a selective manner.
Other aspects, features and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.